Shock Absorbing Concrete (SACON®)
Training is essential for both the military and civilian police forces. However, designing adequate backstops for military training, police, or civilian target practice, has been difficult. There are many difficulties associated with preparing such backstops. For example, a material must be strong enough to stop bullets, yet also must not cause ricochets because of risk to a shooter or bystander and must minimize the creation of lead dust from bullets spalling or fragmenting on impact. Wood backstops are problematic because insect or vermin infestation may lead to degradation and breakdown. Chemical treatment of wood creates additional environmental hazards associated with biocides or other toxins. Earthen barriers are difficult to move and stage in different settings, such as different lighting conditions. Over time earthen barriers may erode or wash away. Historically, fiber reinforced foamed concrete provided some benefits for training structures as a material that (1) resisted breakdown, (2) stopped bullets, and (3) prevented ricochets.
Lead from bullets is another fundamental problem. Lead is a heavy metal neurotoxin that accumulates over time in soft tissues, blood and bone. Exposures to extremely low concentrations of lead have been documented to cause learning disabilities and other neurological damage. Thus, lead is hazard with a potential for long-term harm. Lead from bullets may enter the environment as either elemental lead dust generated from the impact of bullets with the backstop or lead salts which may leach from the backstop into water supplies.
To address many of these concerns, researchers at the U.S. Army Engineer Research and Development Center (ERDC) invented SACON® ballistic concrete, a low-leaching, foamed concrete. SACON® ballistic concrete is effective absorbing low power projectiles and eliminating ricochets. As currently formulated it is a fiber-reinforced concrete with high concentrations of calcium phosphate and aluminum hydroxide to prevent leaching of lead fragments. U.S. Pat. No. 6,264,735 (Bean et al., “the '735 patent”) describes SACON® and the reduced lead-leaching from the SACON® ballistic concrete blocks. U.S. Pat. No. 6,620,236 (Huntsman et al., “the '236 patent”) describes an improvement of the '735 patent formulation that includes an aluminum hydroxide additive to reduce or eliminate the erosion of heavy metals such as lead from the foamed concrete. The contents of both the '735 and the '236 patents are hereby incorporated by reference in its entirety.
SACON® ballistic concrete is prepared using an air compressor by adding a wet foam that contains a foaming agent and a foam stabilizing agent such as hydroxypropyl methyl cellulose. The wet foam is added to the concrete mixture to achieve the appropriate density required by the military specifications. SACON® ballistic concrete has been widely used by the military on bases in the U.S. and abroad. It has been used by other government agencies, e.g., police forces or the Drug Enforcement Agency. It is effective in stopping bullets from conventional small arms such as rifles (0.22 caliber, M16 (5.56 mm)) or pistols (0.38 caliber, 0.45 caliber, and 9 mm) The reported penetration depths range from 1 inch (25 mm) for a 0.38 caliber pistol to 2.55 inches (63 mm) for 5.56 mm (M16 rifle), see Hudson et al., Final Report Demonstration of Shock-Absorbing Concrete (SACON) Bullet Trap Technology, August 1999.
Training facilities use a number of barriers made from precast panels or other shapes to form backstops, simulated buildings, villages, or other structures such as a tank. When combining precast panels together to make a barrier such as a wall or other structure, the precast panel will include features to allow the precast panels to be connected to one another. However, the majority of the precast panel will be bullet absorbing structural components which are intended to capture live fire bullets or projectiles from other munitions such as grenades.
Process for Making Traditional SACON® Ballistic Concrete
Traditional SACON® ballistic concrete is prepared following ERDC specifications. By ERDC specifications it is meant the “Technical Specification for Shock Absorbing Concrete (SACON®)—Shock Absorbing Concrete for Constructing Live-Fire Training Facilities. This 25 page specification with reference to 35 other detailed specifications sets forth in great detail a process for creating a ballistic concrete and using the ballistic concrete to create panels and other structures using the ballistic concrete. The precision of this specification and the requirement for testing involving shooting live ammunition and measuring the depth of penetration of the bullet is a recognition of those in skill in the art of the great difficulties in creating a ballistic concrete that is capable of stopping penetration of a bullet within an acceptable depth without allowing the bullet to ricochet off the surface of the panel.
ERDC Specifications require the following ingredients:
IngredientCubic MeterCubic YardPortland Cement577 kg (1272 lbs.)972 lbs. (441 kg)Fine Aggregate (SSD)577kg (1272 lbs.)972lbs. (441 kg)Water277 kg (611 lbs.)466 lbs. (211 kg)Calcium Phosphate5.78kg (12.7 lbs.)9.72lbs. (4.4 kg)Aluminum Hydroxide5.78kg (12.7 lbs.)9.72 lbs. (4.4 kg)Foam Stabilizer0.15 kg (0.33 lbs.)0.25 lbs. (0.11 kg)Foam (Void System)0.33 m3 (11.7 cu ft.)9.0 cu ft. (0.25 m3)Fiber (choice of)Polypropylene8.8 kg (19.4 lbs.)14.8lbs. (6.7 kg)Steel115 kg (254 lbs.)193lbs. (88 kg)
ERDC requires the use of a foaming agent and foam stabilizing agents. Specifically they require:                Section 2.4.1 Foaming Agents: Foaming agent shall comply with ASTM C 869, tested in accordance with ASTM 796; and        Section 2.4.2 Foam Stabilizing Agents: The stabilizing agent shall contain hydroxypropyl methylcellulose powder limited to 19.0 to 24.0% methoxy and 7.0 to 12.0% hydroxypropoxyl, similar to Dow Chemical Co. K100M.        
Traditional SACON® ballistic concrete uses an air compressor and water to generate pre-formed foam.
Traditional SACON® ballistic concrete specifications require meeting the following density standards:
Density
                (Without fibers) 1442 kg/m3 (90-pcf)        (With polypropylene fibers) 1458 kg/m3 (91-pcf)        (With steel fibers) 1554 kg/m3 (97-pcf)        
The foam is added to the mix, as follows using the ERDC specifications. The void material, pre-formed foam shall be added to the cement slurry to obtain the required density. The material shall be added in increments to reduce the possibility of moving outside the SACON® ballistic concrete density tolerances. The recommended procedure is to add the foam in half increments, i.e., add half of the foam initially by time of insertion and calculate the density. If the density remains above the upper tolerance, add half of the remaining foam and re-calculate the density. If the density continues to remain above the upper tolerance, then add half of remaining foam until the density tolerance of +48 kg/m3 (3-pcf) relative to the target density (based on fiber use) has been achieved.
SACON® ballistic concrete is traditionally installed in modular panels that are typically 24″ to 30″ thick. Panels may be considered compromised when they have been penetrated by greater than 50% of the thickness of the panel, both because of the danger of collapse of the panel and because of the danger of shoot through if the panel is hit at the same point a second time. The panel may be repaired with ballistic concrete or replaced.
FIG. 1 sets forth the prior art process 1000 for making SACON® ballistic concrete.
Step 1004—Create mix without foam.
Step 1008—Create foam (this step could be completed before creating the mix).
Step 1012—Add half the foam to the mix.
Step 1016—Check density of mix plus added foam to see if the density is in range. Note that the target range for density is a pre-fiber target. Anecdotal reports indicate that frequently it is difficult to get the mix into the pre-fiber density range.
Step 1020—Add half of remaining foam if needed to reduce density.
Step 1024—Check density of mix plus added foam to see the density is in desired pre-fiber density range.
Step 1028—If density is too high, go to 1020. If density is now in range, go to 1032.
Step 1032—Add the prescribed fiber and mix.
Step 1036—Test final density. Adding fiber increases the density of the ballistic concrete. Thus, a pre-fiber density of 90 pounds per cubic foot may be increased to a target of 91 pounds per cubic foot when using polypropylene fibers or 97 pounds per cubic foot when using steel fibers.
Step 1040—Branch based on whether the final density has reached the density for the particular fiber. If not, go to 1036. If the density has been reached then proceed to step 1044.
Step 1044—Pour the ballistic concrete into the mold.
Note—Due to the fragility of the foam bubbles within the prior art ballistic concrete mix, the Technical Specifications for Shock Absorbing Concrete (SACON®)-Shock Absorbing Concrete for Constructing Live-Fire Training Facilities has a number of limitations on how the mixed ballistic concrete material is handled. For example, in section 3.6.1 Placement Operations—there is a prohibition against any vertical drops of 2 feet or more absent special authorization and special equipment. The concern from a drop is that a drop larger than 2 feet may destroy a large percentage of the foamed bubbles.
Likewise a horizontal layer shall not exceed more than 2 feet. Here the concern is that the weight of the ballistic concrete on the lower portions of a deep pour would destroy an unacceptable percentage of the foam bubbles. This depth of pour limitation means that wall panels that will be more than two feet tall need to be poured with the height of the final wall in a horizontal plane. This limitation combined with the depth of drop limit precludes pouring a wall in place as the wall cannot be oriented with the height dimension arranged in a vertical orientation. To pour a panel with a thickness of more than two feet, the panel is poured on its side with a trough to lower the ballistic concrete into the form.
Yet another limitation on the treatment of the prior art ballistic concrete material is a requirement that poured SACON® panels are not to be moved for a period of 14 days. The sides of forms may be removed after 3 days. There is a fear of internal cracking in the poured panels if the forms are removed early. Given that wall panels will need to be poured with the long dimensions (wall height and width) placed in a horizontal plane, so that the short dimension (wall thickness) is in the vertical orientation—the large panel sized bottom portions of the mold are tied up for 14 days at a time.
Penetration Test.
The Technical Specification For Shock Absorbing Concrete (SACON®)—Shock Absorbing Concrete For Constructing Live-Fire Training Facilities has a penetration test for ballistic structures. Section 3.3.4.1 Penetration Test reads:                All SACON® wall panels and other cast objects shall be individually tested with a live-fire test of an M855 round fired from an M16A2 rifle at a distance of 25-m (82-ft) and measured for penetration depth to the back of the bullet. Any penetration depth less than 25-mm (1-in.) or greater than 125-mm (5-in.) shall constitute a failure. Those objects failing to meet the penetration depth requirement shall be discarded without further testing or modifications.        For SACON® wall panels and other cast objects that are intended for close quarter usage (distances less than 25-m (82-ft)), the live-fire test shall be conducted with an M855 round fired from a remotely fired M16A2 rifle from the usage distance. Any penetration depth less than 25-mm (1-in.) or greater than 125-mm (5-in.) shall constitute a failure. Those objects failing to meet the penetration depth requirement shall be discarded without further testing or modifications.        
Those of skill in the art will recognize that the term M855 round is the United States military name for the cartridge called the 5.56 NATO round or more specifically, the 5.56 NATO, 62 grain, green tip round. Those of skill in the art will recognize that a bullet is a projectile that is fired from a round (sometimes cartridge) that includes the bullet, casing, propellant, and primer.